Generally, vulcanized rubber is formed by vulcanizing a low-viscosity raw rubber material prepared by natural or artificial synthesis. Such a raw rubber material exhibits excellent flexibility and elasticity, but is problematic in that its forming process is very complicated, its stability is poor, and it is a thermosetting material, and thus it is difficult to reuse. Therefore, there has been a need to develop a novel material for solving the above problem. In accordance with the need, thermoplastic elastomer (TPE) was developed. Thermoplastic elastomers exhibit rubber elasticity under the condition of use, and serve as formable polymer materials under a forming condition. The polymer chain of a thermoplastic elastomer contains an elastic rubber phase (soft phase) and a hard phase preventing plastic deformation at room temperature, such as crystal, glass or the like. Various kinds of thermoplastic elastomers, such as urethane-based elastomer, olefin-based elastomer, amide-based elastomer including the first-developed styrene-based elastomer, have been developed. Such thermoplastic elastomers can be chiefly used in automobile components, electric household appliance components, shoe soles and the like. In the present invention, such a thermoplastic elastomer is used in a cover material for airbags.
An airbag cover must maintain a long-term ornamental appearance in a low-strength zone regardless of temperature change in a vehicle, must have a soft and flexible surface, and must retain sufficient mechanical properties which are not changed by shape, stress, cracking or the like.
Meanwhile, although the shape of an airbag cover is not particularly limited, the feel of the airbag cover to a passenger and the deployment characteristics of an airbag at the time of a collision with another vehicle may be changed according to the raw material of the airbag cover.
An airbag system operates as follows. An impact sensor detects a collision, and then an igniting agent in an expander is electrically or mechanically fired. Due to the firing, heat is generated, and this heat burns a gas generating agent to generate gas. This generated gas is accommodated in the cavity among a holder, an expander and cover, and is charged into an airbag to expand the airbag. Due to the pressure in the expanding airbag, the predetermined area of an airbag cover is broken and deployed to form an opening, and thus an airbag is instantaneously discharged and expanded toward the front of a passenger through the opening. The expanded airbag acts as an impact absorber, thus preventing a passenger from being injured by a collision. Therefore, an airbag cover is necessary for an airbag system, and is required to allow an airbag to be instantaneously discharged and expanded by accurately deploying the airbag without forming airbag cover fragments injuring a passenger when an airbag system operates and thus generates gas due to the occurrence of collision.
At the time of usage and operation of an airbag, the performance of an airbag cover is very important. Since an airbag cover may be actually subject to various temperature conditions, its performance must be guaranteed under extremely severe conditions. Thus, generally, when an airbag module is unfolded at a low temperature of −35° C., the predicted region of an airbag cover must be torn and opened, and the fragments of the airbag cover must not be shattered due to the breakage of the airbag cover. Further, when an airbag module is unfolded at a high temperature of +85° C., an airbag cover must not be deformed and stripped by heat.
When a vehicle operates in a normal state, an airbag cover serves to store an airbag cushion. When an airbag cover has enough impact strength to open an airbag module, an airbag cushion must be opened in 1/1000 of a second, and an airbag cover molded product designed for this purpose allows an airbag cushion to be normally unfolded. During the unfolding procedure, an airbag cover serves to unfold an airbag cushion, and other parts serve to hold an airbag cushion and other elements while being attached to a steering wheel.
The composition of such an airbag cover requires excellent impact strength at low temperature, and this performance is required at lower temperature.
Korean Patent Application Publication No. 10-2008-0005352 discloses a polyolefin composition for manufacturing a cover for mounting an airbag, wherein the composition is crosslinked by applying a crosslinking agent and a free radical initiator, thus improving the low-temperature shock resistance thereof.
In conventional technologies, organic peroxide, which is used as a crosslinking agent, acts on the chains of polypropylene and a polyolefin-based thermoplastic elastomer to form a network together with a free radial initiator and a co-crosslinking agent, thus obtaining a crosslinking effect. However, due to the form of organic peroxide, the chains of polypropylene and a polyolefin-based thermoplastic elastomer are cut irregularly to cause an undesired molecular weight reduction effect, thus deteriorating physical properties.
Further, in the network formed by a crosslinking agent, a free radical initiator and a co-crosslinking agent, there is a limitation in precisely controlling degree of crosslinkage to realize the physical properties of a resin composition, and there is a problem that a crosslinking agent and a free radical initiator remaining in the form of organic peroxide without participating on a crosslinking reaction deteriorates the thermal stability of a resin composition. Due to the deterioration of physical properties of the resin composition, the low-temperature impact resistance of an airbag cover deteriorates, so that, at the time of unfolding an airbag, the desired area of an airbag cover is not torn, and the undesired area thereof is torn because the air bag cover is separated in an unintentional direction.
Therefore, the present inventors have completed a composition having improved low-temperature impact strength, tensile strength and tensile elongation by partially crosslinking the composition without using a crosslinking agent, a co-crosslinking agent and a free radical initiator.